The present invention relates to a method for manufacturing alkali chromates by the reacting of chromium ore with alkali compounds and oxidation with oxygen-containing gases.
It is known to react chromium ore by roasting with alkali compounds to alkali chromates at temperatures of 900.degree. to 1100.degree. C. The reacting takes place conventionally in directly heated revolving tubular kilns. The reaction proceeds, depending on the alkali compound used, mainly according to the following overall reaction equations (1), (2) and/or (3): EQU 4 FeCr.sub.2 O.sub.4 +8 Na.sub.2 CO.sub.3 +7 O.sub.2 .fwdarw.8 Na.sub.2 CrO.sub.4 +2 Fe.sub.2 O.sub.3 +8 CO.sub.2 ( 1) EQU 4 FeCr.sub.2 O.sub.4 +16 NaOH+7 O.sub.2 .fwdarw.8 Na.sub.2 CrO.sub.4 +2 Fe.sub.2 O.sub.3 +8 H.sub.2 O (2) EQU 4 FeCr.sub.2 O.sub.4 +16 NaHCO.sub.3 +7 O.sub.2 .fwdarw.8 Na.sub.2 CrO.sub.4 +2 Fe.sub.2 O.sub.3 +16 CO.sub.2 +8 H.sub.2 O (3)
Difficulties arise in the carrying out of this reaction because of the fact that intermediately low-melting mixtures are obtained, which on the one hand lead to sticking of the reaction mixture to the reactor walls and to granulation and on the other hinder the access of oxygen into the reaction compound, so that the reaction occurs only with limited space/time yield and incompletely.
Improvements to the method concern in particular the addition of leaning materials to the reaction mixture, in order on the one hand to prevent the sticking to the reactor walls and the granulation and on the other also to facilitate the access of oxygen. Other method improvements concern the raising of the oxygen content in the oxygen-containing gas, see e.g. U.S. Pat. No. 3,095,266, U.S. Pat. No. 3,733,389, U.S. Pat. No. 4,162,295 and U.S. Pat. No. 4,244,925. The use as oxygen-containing atmosphere of pure oxygen has also already been considered. In this case a direct or indirect electrical heating of the roasting kiln or an indirect heating with burner is necessary. However, very high demands have to be placed on the thermally conductive materials for the kiln construction, so that preferably revolving tubular kilns heated directly by the burning of carbon-containing materials have continued to be used (U.S. Pat. No. 4,244,925, column 4, lines 56 to 62), wherein the mixing of the oxygen-containing gases with the burner waste gases has been accepted. Moreover it is necessary according to the known methods to pass through the kiln far more oxygen than is necessary for the oxidation reaction, in order to maintain a high oxygen content in the kiln atmosphere. Even with indirect heating of the revolving tube and feeding of pure oxygen according to WO 91/17 118 a rarefaction of the kiln atmosphere as a result of the gases released during the calcination is accepted.
There have already been proposals (DE patent 544 086) that the reaction given above according to equation (1) be separated locally into two stages, wherein in the first stage mainly CO.sub.2 is released and the oxidation takes place mainly in the 2nd stage, so that leaning material-free operation is possible.
According to DE 544 086 as low a temperature as possible in the range from 800.degree. to 900.degree. C. is to prevail in the calcination stage, in order that leaning material-free operation is possible. A further heating in the oxidation stage to 900.degree. to 1100.degree. C. is therefore necessary, so that the oxidation takes place in an oxygen atmosphere rarefied by the burner waste gases at a relatively slow oxidation rate.